Question

x²-4x-21=0 como resolver??????​

Answer 1

Explicação passo-a-passo:

$\sf x^2-4x-21=0$

$\sf \Delta=(-4)^2-4\cdot1\cdot(-21)$

$\sf \Delta=16+84$

$\sf \Delta=100$

$\sf x=\dfrac{-(-4)\pm\sqrt{100}}{2\cdot1}=\dfrac{4\pm10}{2}$

$\sf x'=\dfrac{4+10}{2}~\Rightarrow~x'=\dfrac{14}{2}~\Rightarrow~\red{x'=7}$

$\sf x"=\dfrac{4-10}{2}~\Rightarrow~x"=\dfrac{-6}{2}~\Rightarrow~\red{x"=-3}$

$\sf S=\{-3,7\}$